The NaCl and water absorption across the eel intestine was inhibited by serotonine (5-HT), acetylcholine (ACh) or atrial natriuretic peptide (ANP), ANP being the most potent inhibitor. The effects of ANP were completely abolished after pretreatment with 8BrcGMP, indicating that ANP acts through cGMP synthesis. Similarly, 5-HT seems to activate cAMP synthesis, since the inhibitory effects of 5-HT were significantly reduced after pretreatment with 8BrcAMP. ACh seems to enhance the intracellular Ca^<2+> concentration, however the effect of Ca^<2+> ionophore, ionom-ycin, was not sufficient to mimic the effect of ACh, suggesting involvement of other mediators.When the intestine was stimulated electnically, the Cl^- transport was reduced transiently. The transient response was inhibited by 70% with atropine, a muscarinic ACh antagonist, and ICS-205930, a 5-HT antagonist, indicating that 70% of the effect of field-stimulation is due to release of ACh and 5HT. The remainig 30% must-be due to other regulators.On the other hand, adrenaline (AD) enhanced the NaCl and water absorption after pretreatment with 5-HT and ACh. The effects of AD were completely abolished with yohimbine, alpha_2-receptor antagonist. Moreover, the effects of AD were still observed even in the presence of 8BrCAMP and ionomyein, suggesting that AD acts on a post-cAMP and post-Ca^<2+> process.A novel peptide was also extracted from the eel intestine, but had no effect on the ion and water transport. The eel intestinal pentapeptide (EIPP, H-Gly-Phe-Trp-Asn-Lys-OH) enhanced the contraction of the srnooth muscle in the digestive tract such as esophagus, stomach and intestine.